FIG. 1 depicts a wireless network 100 consisting of several stations 101, 102, 103 and 104. Depending on the kind of network i.e. infrastructure or adhoc, the wireless network may provide its stations with connectivity to other networks, wireless or otherwise. In an infrastructure network, this connectivity is typically achieved by an Access Point (AP). In the example of FIG. 1, station 101 is arbitrarily designated as the AP. Adhoc networks typically exist for the local transfer of data between a plurality of co-located devices. As such, typically adhoc networks do not have a link to external networks.
Medium Access Control
Medium Access Control (MAC) architectures are fundamentally of two different types—centralized control and distributed control. In centrally controlled systems, one of the stations in the network is responsible for allocating channel capacity to individual stations and coordinating access to the channel. In infrastructure networks, the AP 101 typically handles this network coordination functionality. In adhoc networks, one of the stations or nodes typically assumes the role of network coordinator and performs this function.
In centrally controlled networks, the network coordinator typically schedules the use of the available channel resources depending on the requirements of different stations. Stations then confirm to this schedule, which is either announced by polling as per the Point Coordination Function (PCF) mechanism specified in IEEE 802.11 standard (“Local and Metropolitan Area Networks—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications”, IEEE Std802.11-1999, IEEE, August 1999: hereinafter referred to as Non-Patent Document 1) or by a-priori schedule announcement and timing synchronization as in the HIPERLAN/2standard (“Broadband Radio Access Networks (BRAN) HIPERLAN Type 2 Data Link Control (DLC) Layer”, ETSI TS 101 761-1 v1.1.1, April 2000: hereinafter referred to as Non-Patent Document 2).
Stations effecting communication using distributed channel access mechanisms typically operate as peers whereby each station contending to transmit data on the medium performs a random backoff in order to reduce the probability of collision with other stations, and thereby increase the effective throughput of the network. The Distributed Coordination Function (DCF) specified in the IEEE 802.11 standard (Non-Patent Document 1) is an example of such a system. Enhancements to the basic DCF access mechanism include prioritized access for certain traffic classes which are realized by means of differentiated medium access delays. An example of this is embodied as the Hybrid Coordination Function Contention Based Channel Access (HCCA) in the draft IEEE 802.11e specification (“Draft Supplement to LAN/MAN Specific Requirements—Part 11: Wireless Medium Access Control (MAC) and Physical Layer (PHY) specifications: MAC Enhancements for Quality of Service (QoS)”, IEEE Std 802.11e/D7.0 January 2004: hereinafter referred to as Non-Patent Document 3).
While centralized channel access has been shown to be beneficial in delivering data that is of a recurring/periodic nature and requires a certain level of Quality of Service (QoS), examples being audio-visual (AV) and voice streams, distributed channel access mechanisms are found to be more efficient for non-recurring data such as http and ftp traffic, which are of a more bursty nature. As such, standards such as the IEEE 802.11 (Non-Patent Document 1) and the ongoing 802.11e draft (Non-Patent Document 3) cater to both kinds of traffic by specifying channel access mechanisms that cater to both stream-like and bursty traffic.
Carrier Sense Multiple Access
Conventional contention based channel access utilizes the well-studied Carrier Sense Multiple Access (CSMA) protocol. According to the CSMA protocol, a station wishing to transmit data senses the medium using a Clear Channel Assessment (CCA) algorithm as is specified in Non-Patent Document 1. Upon detecting the medium to be idle, the station waits for a minimum mandatory amount of time during which the medium must remain idle, prior to choosing a random backoff. The random backoff is decremented at fixed slot intervals while observing the medium to be idle.
As stated previously, stations are required to wait for a constant duration of time before choosing a random backoff. This duration accounts for certain physical layer processing overheads such as CCA and processing delays. At the MAC level, variations in this constant duration are intended to provide prioritized access to a response frame, or to stations with higher priority such as the access-point (which generally have the ability to preempt other stations in attempting to access the medium), or to stations with traffic of a higher priority.
Upon decrementing the random backoff to zero, the station initiates its transmission. As in a distributed environment, several stations simultaneously contend to transmit on the medium, there exists a finite probability that two stations may attempt to simultaneously transmit, resulting in a collision and a net reduction in the network throughput.
FIG. 2 uses the 802.11 standard (Non-Patent Document 1) as an example to illustrate a scenario in which a station attempts access on the medium using the contention based DCF access mechanism. Upon detecting the medium to be idle and with data to transmit at time instant 202, the station starts waiting for a constant duration of idle time, referred to as the DCF Inter-Frame Space (DIFS) 205. The Figure also marks out the Short Inter-Frame Space (SIFS) 203 that is used for response or continuation frames, and the PCF Inter-Frame Space (PIFS) 204 that is used by the AP to preempt medium access. As explained previously, the SIFS and PIFS are used to provide higher priority to response frames and the access point respectively. In addition to the SIFS and PIFS, the 802.11e draft specification (Non-Patent Document 3) specifies differentiated levels of DIFS in the form of Access Category Inter-Frame Spaces (AIFS). These are primarily targeted at providing differentiated service to different traffic categories.
Upon completion of the DIFS interval, the station chooses a random backoff value, and begins to decrement it every slot time, while the medium is idle 206. At time instant 207 another station transmits 208 on the medium, resulting in the CCA indicating to the station that the channel is busy and resulting in deferral of the backoff decrement procedure. Once the other station has completed its transmission at time instant 209, the original station detects the medium to be idle, waits for DIFS duration 210 and decrements the remainder of its backoff counter 211 to zero, initiating its transmission 213 at time instant 212.
Virtual Carrier Sensing
As wireless devices are unbounded and tether-less, wireless networks often encounter the problem of hidden-nodes—i.e. a device that is within radio-range of the recipient station may not be within the radio-range of the transmitting station. An example of this is a scenario that results in a node “hidden” from the transmitter not being able to receive the transmitted frame. As a result of this, the “hidden station” may initiate a transmission of its own, resulting in a collision at the receiver.
To counter the problem of hidden nodes the IEEE 802.11 standard (Non-Patent Document 1) specifies a virtual carrier sensing mechanism. In addition to the CSMA/CA technique, stations maintain a Network Allocation Vector (NAV), which is an indicator of periods of expected activity on the medium. A station with its NAV set will not initiate a transmission, irrespective of the medium state, as determined by its carrier sensing mechanism.
Stations receiving a transmission use information contained in the duration field of the frame to set their NAVs. The duration field of a frame typically includes the duration required to complete any expected response frame.
Another mechanism designed to facilitate the virtual carrier sensing mechanism is the Request-To-Send/Clear-To-Send (RTS/CTS) frame exchange sequence specified in Non-Patent Document 3. FIG. 3 depicts an example in which a transmitter, having gained access to the medium at time instant 301, precedes its transmission with an RTS frame 302. The duration value specified in the RTS frame “protects” the intended transmission and its acknowledgement by setting the NAV 303 of all recipient stations up to the instant of time 309. The recipient of the RTS responds one SIFS 304 later, with a CTS frame 305, which sets the NAV 306 of receiving stations up to the instant of time 309. As a result of successful transmission of the RTS and CTS frames all stations within range of both the transmitter and receiver will not initiate transmission up till the time-instant 309 there by allowing the data 307 and acknowledgement 308 transmissions to proceed with a reduced probability of error due to collisions. Upon completion of the frame exchange sequence, stations need to wait for a DIFS 312 duration of medium inactivity before attempting access to the medium at time instant 313. The access point, having a higher priority, may access the medium after a PIFS duration 310 at time instant 311.
The NAV procedure described above proves expensive in terms of bandwidth on the medium in scenarios where for some reason a station that initiated the RTS-frame, fails to transmit after the CTS is received. Such a scenario would result in the duration of time protected by the NAV in going unutilized. To counter scenarios where the medium would be otherwise unutilized Non-Patent Document 1 provides for NAV reset procedures.
In the scenario described above, stations that used the RTS-frame to set their NAV may reset their NAV i.e. initiate access to the medium if their PHY layer modems do not indicate medium activity at the point where the originator of the RTS is expected to access the channel. After the timeout interval, stations undergo a NAV-reset and wait for a DIFS duration followed by a random backoff before transmitting. Likewise, the station initiating the RTS may access the medium by performing a backoff procedure if after a CTS time-out the CTS is not received. Similar NAV reset procedures are defined for stations that set their NAV based on duration values indicated in a fragment of a frame exchange sequence.
In prior art such as described in Non-Patent Document 1, at least three independent inter-frame timings, namely—SIFS, PIFS and DIFS are required in order to facilitate differentiation between response/continuation frames, pre-emption by the access point and conventional channel access by stations. In Non-Patent Document 3 further differentiation is provided for traffic of stations performing conventional channel access by replacing the DIFS with four different AIFS inter-frame spaces.
In a moderately loaded medium it is quite conceivable that a station would need to backoff several times (due to transmission pre-emption by other stations) before it succeeds in making a transmission. Under the DCF protocol, each successive backoff involves waiting for a DIFS duration before re-initiating the decrement process of the backoff counter.
In one of its embodiments the current invention describes a method to reduce the inter-frame space by facilitating a means to dynamically alter the interpretation of the inter-frame space by means of the methods laid out in this text.